refactor(animation): Replace enums with string literal type aliases

[acdvorak]

Refs #4225

IMO this makes the code a little easier to read/write.

https://www.typescriptlang.org/docs/handbook/advanced-types.html#string-literal-types

[moog16]

Sleeping on this one - string literals might be cleaner, but the enums also provide an easy avenue to refactoring. In this case I don't think it matters as much, since the naming of animationend or transform are going to change :)

[kfranqueiro]

I for one vastly prefer this approach... but it looks like it's failing tests in its current state.

[moog16]

My only reason for using enums were for autocomplete, and avoiding spelling mistakes. I think in this case though, these can already be autocomplete'd. If you have a stronger opinion here then lets go with yours 👍

[codecov-io]

Codecov Report

Merging #4287 into feat/typescript will decrease coverage by <.01%.
The diff coverage is 100%.

@@                 Coverage Diff                 @@
##           feat/typescript    #4287      +/-   ##
===================================================
- Coverage            98.37%   98.36%   -0.01%     
===================================================
  Files                   93       93              
  Lines                 5771     5736      -35     
  Branches               778      778              
===================================================
- Hits                  5677     5642      -35     
  Misses                  94       94

Impacted Files	Coverage Δ
packages/mdc-animation/index.ts	100% <ø> (ø)	⬆️
packages/mdc-tabs/tab-bar/foundation.js	100% <100%> (ø)	⬆️
packages/mdc-slider/foundation.js	100% <100%> (ø)	⬆️
packages/mdc-checkbox/index.js	98.79% <100%> (-0.02%)	⬇️
packages/mdc-tabs/tab-bar-scroller/index.js	100% <100%> (ø)	⬆️
packages/mdc-linear-progress/foundation.ts	100% <100%> (ø)	⬆️

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[mdc-web-bot]

All 621 screenshot tests passed for commit 0fd0915 vs. feat/typescript! 💯🎉

[mdc-web-bot]

All 621 screenshot tests passed for commit 17bee8e vs. feat/typescript! 💯🎉

[moog16]

one comment, but after that it looks good.

[mdc-web-bot]

All 621 screenshot tests passed for commit 9a3a101 vs. feat/typescript! 💯🎉

[mdc-web-bot]

All 621 screenshot tests passed for commit 439e25f vs. feat/typescript! 💯🎉